fix(compiler-sfc): support global selector nesting

[yangxiuxiu1115]

fix #12404
fix #12405

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	100 kB	37.9 kB	34.2 kB
vue.global.prod.js	158 kB	57.8 kB	51.4 kB

Usages

Name	Size	Gzip	Brotli
createApp (CAPI only)	47.2 kB	18.3 kB	16.8 kB
createApp	55.2 kB	21.3 kB	19.5 kB
createSSRApp	59.3 kB	23.1 kB	21 kB
defineCustomElement	60.1 kB	22.9 kB	20.8 kB
overall	69.1 kB	26.5 kB	24.1 kB

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commit: 436415a

[edison1105]

The original behavior was that global would ignore the preceding rules, but it will no longer do so after this PR. this will introduce some edge cases. see:

• Playground with main
• Playground with this PR

[skirtles-code]

I've been experimenting with this and I'm wondering about some apparent inconsistencies in how selectors are handled. For example:

• Playground

The background-color is applied to some of the headings, but not others. It's not really clear to me what would be considered correct here, but I can't really justify the differences.